Optical lithography is a standard process for patterning state-of-the-art devices in the semiconductor industry. Extensions of current lithographic techniques to form smaller features, and thus faster processors, are currently being explored. Advanced lithographic schemes currently undergoing research include the use of short wavelength radiation (e.g., wavelengths below 200 nm) to allow patterning of smaller and smaller features on semiconductor substrates such as silicon wafers.
Impediments to utilizing short wavelength radiation in optical lithography include identifying potential compositions that can be effectively utilized as masks with such actinic radiation to form patterns on substrates with acceptable line resolution. Indeed, it is clear that mask compositions (e.g., photoresists) that can be effectively utilized at one wavelength are not necessarily effective at another wavelength.
For example, with respect to the problem of radiation absorption, the development of polyhydroxystyrene based resists was necessary to overcome high absorbance of phenolic polymers, e.g., novolac resists at 248 nm, and enabled the introduction of 248 nm lithography into integrated circuit manufacturing. In a similar manner, new polymer systems are needed to overcome the high 193 nm absorbance of phenolic-based polymers. Two different classes of resists, based on either polyacrylate or polycyclic copolymer compositions, have been developed to provide resists with decreased absorption relative to phenolic-based polymers.
However, due to the high absorbance of polyhydroxystyrene, polyacrylate, and polycyclic copolymer based resists, the use of any of these compositions at very short wavelengths remains problematic unless the coated resist thickness is under 100 nm. This has led to the development of fluorinated polymers as potential resist materials for 157 nm lithography. These examples, among others, show that the effectiveness of resist materials at one wavelength cannot be used to generally show its effectiveness at another remote wavelength.
Even with the development of some photoresists that potentially have appropriate radiation adsorptive properties for lithography, other problems persist. For example, pattern formation on a photoresist conventionally utilizes aerial imaging, which projects an optical image upon a photoresist with contrasting exposure areas to activate portions of the photoresist that are sufficiently illuminated by actinic radiation. Unfortunately, the modulation transfer function of such an aerial image, which quantifies the intensity of the radiation as a function of position, is not a perfect step function, i.e., the light intensity cannot change instantaneously from one finite value to another over an infinitesimally small length scale. As a result, the modulation transform function typically varies continuously, with a steeper slope corresponding with larger potential contrast that can be achieved over a given distance. As smaller and smaller features are desired, without a change in optics or other equipment, the modulation transfer function virtually “flattens,” i.e., the change in contrast over a given distance needs to be steeper for smaller and smaller features. When the slope is too shallow, contrast distinctions for line features are difficult to achieve. Thus, line resolution is effectively limited as smaller and smaller features are desired.
Liquid immersion lithography has the potential to provide better line width resolution at a given actinic radiation wavelength. Though it would be potentially advantageous for liquid immersion lithography to utilize the same types of mask compositions as employed in dry lithography, concerns exist regarding the potential leaching of chemicals from the photoresists into the liquid. Such leaching can detrimentally affect the resolution of the optical system by changing the index of refraction of the liquid. Furthermore, leaching can potentially result in optical lens contamination, which, beyond potentially affecting optical performance, can result in time-consuming equipment maintenance or more frequent optics replacement.
Contrast enhancement is another microlithography technique that can extend the practical limits of optical lithography by potentially enhancing the contrast of a pattern formed on a photoresist. A contrast enhancement layer (CEL) is a photobleachable film that can be applied as a conformal mask over a photoresist. The CEL initially has a high absorbance for the actinic radiation exposure wavelength, but becomes more transparent upon accumulated radiation exposure, allowing more of the actinic radiation to pass onto the underlying photoresist. Though some CEL materials have been developed for G-line (436 nm), I-line (365 nm), and broad-band lithography, the development of a CEL at more advanced wavelengths of 248 nm and below has been limited by the inability to find a material that is capable of high initial absorbance, high bleaching rates, and low final absorbance. This is particularly true of wavelengths below 200 nm (e.g., 193 nm and 157 nm).
Accordingly, a need exists for methods and compositions that can improve or enhance the contrast of patterns (e.g., improving line resolution) formed on photoresists using short wavelength lithography (i.e., utilizing actinic radiation with wavelengths below 200 nm).